NORMALLY PRUDENT people are suggesting a summit of dimensions that would have been dismissed as delusionary not so long ago. A good treaty eliminating whole classes of missiles will be signed. The political momentum it generates and the verification precedent it sets make possible an early approach to reducing both conventional and strategic arms -- reducing the latter, says Mr. Reagan, by a "gigantic" measure. A formula allowing each side's work on strategic defense to proceed, and without prejudice to accord on strategic offense, is being sought. Soviet spokesmen predict Soviet withdrawal from Afghanistan next year. The human rights account is active, and the shape of future, expanded economic relations is under review.

The prospect with which Moscow now tantalizes the West arises in the first instance from Mikhail Gorbachev's felt requirement to tend domestic economic and social problems that were so aggravated they could no longer be ignored. The bold reformist vision embodied in his "restructuring" program has yet to move far off paper; it has engaged the Soviet elite but not yet won the bureaucracy or rewarded the people. To pursue it effectively, however, he needs, among other things, a respite from the expensive, risky, superheated arms and Third World competitions to which his country contributed disproportionately in the past decade. The effort raises the fundamental question of whether, by intent or circumstance, the respite may lead to a new round of international confrontation or to a settling down over a longer term.

The sense of big and exciting stakes has to be balanced against awareness of the disappointments experienced at seemingly similar moments in the past. A Soviet newspaper said the other day, "the random nature of the U.S. domestic political struggle" remains the "most difficult and permanent obstacle" to Soviet-U.S. discourse: they look at our politics and -- given the nature of their own -- they predictably don't get it. On their part, many Americans are skeptical of the depth and permanence of Mr. Gorbachev's apparent conversion to a more reasonable outlook. They ask whether his personal understanding of America is deep enough, and his political situation secure enough, to sustain a new course.

Undeniably, however, a moment of high drama is here. Ronald Reagan contributed to it by a policy of steadiness tempered now by an openness to accommodation -- an openness many Americans doubted he had in him. Mikhail Gorbachev undertook to look again at some Soviet notions long set in stone. These unlikely partners meet in circumstances favorable to a thoroughgoing test, over the next few days and over the next year or more, of the potential of improving Soviet-American ties